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Ocean acidification, and effect on biological calcification

Information below is mainly drawn from One of the most import papers in this field is the one by Orr et al. (2005), published in Nature (Orr_OnlineNature04095.pdf)

Increased CO2 concentrations lead to reduced pH in the oceans, and hence to reduced CO32- concentrations.

CO2 (aq) + H2O  H2CO3  HCO3− + H+  CO32− + 2 H+

With lower pH, and thus higher [H+] this equilibrium will first be pushed to HCO3-, then to CO2. The saturation state for calcium carbonate is described by

Ω = ([〖Ca〗^(2+) ][〖CO〗_3^(2-)])/K_sp 

If Ω < 1, the mineral dissolves; at >1, it can grow. Ca2+ is relatively constant throughout the oceans ( Since Ksp is a constant, the behavior of the saturation is mostly driven by concentration of CO32-

Due to temperature, pressure and depth, there is a natural horizontal boundary, the ‘horizon’, below which the calcium carbonate is below saturation level, above Ω > 1. With increasing acidification, this horizon raises to the top, and more and more calcifying organisms are in zones where it becomes the longer the more difficult to calcify their skeletons.

There are two forms of calcium carbonate, calcite and aragonite, that are used by ‘calcifying’ organisms. Aragonite dissolves a bit easier than calcite, so the saturation horizon of calcite is a bit lower than the one for aragonite. Also, there are two forms of calcite: high-magnesium calcite dissolves easier than low-magnesium calcite.

Main calcifying groups in the oceans are coccolithophores, corals, forams, echinoderms, crustaceans (including ostracoda, or seed shrimp; wikipedia, EoL), molluscs (including pteropods or pelagic molluscs, the ‘sea butterflies’: wikipedia, EoL). Different groups use different forms of calcium carbonate:

  • Aragonite: corals; nacre of mollusks; pteropods
  • Calcite: coccolitophores, forminifera, crustacean (latter mostly high-magnesium calcite, but also several other forms)

Coral reefs are very sensitive to aragonite saturation, and are only found in areas with Ω (aragonite)>3; areas with 3-3.5 are marginal; 3.5: adequate for growth; >4: development of coral reefs.

Article on effects of crystal type on effects of acidification: Kroeker_2011_Ecol_Lett_Response.pdf and references in there.

Crustaceans seem to be very robust against the effects of ocean acidification, and we assume the ostracods are no exception here (but will try and get confirmation); pteropods are known to be very sensitive. Having data on these two groups and doing a comparative Species Distribution Modelling exercise should be interesting.

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